liquid-retaining structures

Introduction

This guidance is based on Eurocodes BS EN 1992-1-11_and

BS EN 1992-32_{and the corresponding UK National Annexes. This is}

not an exhaustive treatment of the subject and the reader is advised to refer to The Concrete Centre book on concrete basements3_and

CIRIA publication C6604_{as basements and tanks have many aspects in}

common. Watertightness

In the design of containment structures, liquid tightness is a critical consideration. The structural engineer should discuss and agree with the client the liquid tightness requirements. Colloquial phrases such as ‘waterproof’ construction are best avoided; instead the engineer should agree the degree of leakage that can be tolerated using the classification of tightness classes shown in Table 1. This, in turn, will lead to limiting crack widths that should be used in structural design. Tightness class 1 is the most usual class for utilitarian water-retaining structures and limiting crack widths is normally sufficient to achieve this class. For tightness classes 2 and 3 it will not be sufficient to limit crack widths alone. Liners and/or prestressing will be required to meet the requirements.

In addition to correct design, liquid tightness also depends on the use of an appropriate concrete mix and good workmanship on site. Good compaction of concrete is essential.

Durability and selection of materials

Concrete should be specified in accordance with BS EN 2065,6_and

BS 8500 Parts 1 and 27,8_{. All materials in contact with potable water}

will need to comply with specific regulations. The Civil Engineering Specification for the Water Industry9_{provides useful information.}

Well-compacted concrete is essential for durability. Generally, the thickness of members should be at least 250mm to permit good compaction. However, the thickness should not be excessive as the reinforcement required increases with the thickness of the concrete. The likely exposure classes for different elements are noted in

Table 2. Cover requirements in BS EN 1992-1-1 and BS 8500 will generally apply. It is good practice to use nominal cover c_nom of 45mm from the face in contact with liquid and 75mm from any face cast against soil.

Concrete mix

In liquid-retaining structures, mix design should aim at durability and minimising the risk of cracking. Strength is rarely critical. Watertightness and durability can be achieved using good-quality concrete alone without any special additives or admixtures.

The following specification is likely to be satisfactory for most cases: • Consistence class: S3

• Maximum water cement ratio: 0.50

• Minimum cement content: 300kg/m3_{when aggregate size is 20mm;}

• 320kg/m3_{when aggregate size is 14mm}

• Maximum cement content: 400kg/m3_{for CEM I (OPC) concrete and}

450kg/m3_{when ground granulated blast-furnace slag (GGBS) or fly}

ash is used

• Concrete strength class: C30/37

Use of cement replacement (GGBS or fly ash) is recommended as the heat of hydration will be less than that for pure Portland cement (CEM I) and this in turn assists in crack control. Suitable cement or combination types are CEM IIB-V (which contains 21–35% fly ash) or CEM IIIA (which contains 36–65% GGBS). If high proportions of cement replacements are used, there will be implications for early strength and abrasion resistance, which might have an effect on the programme.

This article highlights some of the key

considerations for the design and construction

of liquid-retaining structures.

Figure 1 Aeration and clarifier tanks at New Hythe, Kent

›

TheStructuralEngineer 44 Technical Number 1 Concrete design January 2015 Introduction

This short note highlights some of the salient aspects of the design and construction of liquid-retaining structures in reinforced concrete

(Figure 1). This guidance is based on Eurocodes BS EN 1992-1-11_and

BS EN 1992-32_{and the corresponding UK National Annexes. This is}

not an exhaustive treatment of the subject and the reader is advised to refer to The Concrete Centre book on concrete basements3_and

CIRIA publication C6604_{as basements and tanks have many aspects in}

common.

Watertightness

Tightness class 1 is the most usual class for utilitarian water-retaining structures and limiting crack widths is normally suffi cient to achieve this class. For tightness classes 2 and 3 it will not be suffi cient to limit crack widths alone. Liners and/or prestressing will be required to meet the requirements.

In addition to correct design, liquid tightness also depends on the use of an appropriate concrete mix and good workmanship on site. Good compaction of concrete is essential.

Durability and selection of materials

Concrete should be speciﬁ ed in accordance with BS EN 2065,6_and

BS 8500 Parts 1 and 27,8_{. All materials in contact with potable water}

will need to comply with speciﬁ c regulations. The Civil Engineering Speciﬁ cation for the Water Industry9_{provides useful information.}

The likely exposure classes for diff erent elements are noted in

Table 2.

Cover requirements in BS EN 1992-1-1 and BS 8500 will generally apply. It is good practice to use nominal cover Cnom of 45mm from the face in contact with liquid and 75mm from any face cast against soil.

Concrete Design Guide

No. 1: Guidance on the design of

Concrete mix

The following speciﬁ cation is likely to be satisfactory for most cases:

Consistence class: S3

Maximum water cement ratio: 0.50

Minimum cement content: 300kg/m3_{when aggregate size is 20mm;}

320kg/m3_{when aggregate size is 14mm}

Maximum cement content: 400kg/m3_{for CEM I (OPC) concrete and}

450kg/m3_{when ground granulated blast-furnace slag (GGBS) or ﬂ y ash}

is used

Concrete strength class: C30/37

Use of cement replacement (GGBS or ﬂ y ash) is recommended as the heat of hydration will be less than that for pure Portland cement (CEM I) and this in turn assists in crack control. Suitable cement or combination types are CEM IIB-V (which contains 21–35% ﬂ y ash) or CEM IIIA (which contains 36–65% GGBS). If high proportions of cement replacements are used, there will be implications for early strength and

�

Figure 1

Aeration and clariﬁ er tanks at New Hythe, Kent

HO WE S A TKINSON CRO WDER

This series is produced by The Concrete Centre to enable designers to realise the potential of concrete.

The Concrete Centre, part of the Mineral Products Association (MPA), is a team of qualiﬁ ed professionals with expertise in concrete construction, engineering and architecture. www.concretecentre.com

www.concretecentre.com I 41

Structural Design of Concrete and Masonry

When in contact with aggressive soil, provisions to resist sulphate attack are likely to control the mix and a cement type CEM IIIB or CEM IVB could be used.

Where chemically aggressive liquids are stored, expert guidance should be sought for the selection of appropriate concrete mix9_.

Normally, protective liners will be required. Basis of structural design

Design situations

These are dealt with in a general way in BS EN 199010_{. For tanks}

constructed partially or fully below ground:

• adverse effects of soil and groundwater pressures on the walls and base should be considered during construction and in service; this will normally require consideration of the tank when it is empty • for the design situation when the tank is full, no relief should be

given for the beneficial soil and groundwater pressure effects Actions on liquid containment structures

Permanent actions

Common permanent actions to be considered are: • self-weight of tank and contents

• weight of plant and equipment Variable actions

Common variable actions to be considered are: • loads due to liquid pressures

• wind loads on structures at or above ground • snow loads on covered structures

• uplift forces on underground tanks due to ground water

• lateral loads due to earth and water pressures on underground tanks

Values of actions

Values of actions should be established using the relevant codes. In practice the designer is likely to know only the maximum depth of liquid that it is physically possible to store. The operational depth will be slightly smaller. It is recommended that all calculations are carried out using the full depth of the tank. Although slightly conservative, this approach will result in a reliable design.

Partial factors on actions

Partial factors for permanent and variable actions are given in the UK NA to BS EN 1990. Tables 3 and 4 show the partial factors recommended for loads and pressures induced by retained liquids, noting that designs should be based on the assumption that the maximum liquid level would be the top of the walls.

Structural analysis

Design should be based on elastic analysis without redistribution. In rectangular tanks, direct tension in the plane of the walls arises from the lateral load supported by adjacent contiguous walls and this should be taken into account in design. Hoop stresses in circular tanks also lead to in-plane tension.

Structural design – ultimate limit state

Guidance in BS EN 1992-1-1 should be used. The following should be noted in connection with the calculation of shear resistance:

• the shear resistance of a section not reinforced for shear V_Rd,cshould be calculated making allowance for the presence of any tension

Figure 2 Tensile strength and stress over time

Table 1: Liquid tightness classes*

Tightness class Requirements for leakage

0 Some degrees of leakage acceptable or leakage of liquids irrelevant 1 Leakage to be limited to small amount. Some surface staining or damp

patches acceptable

2 Leakage to be minimal. Appearance not to be impaired by staining 3 No leakage permitted

*Based on BS EN 1992-3

Table 2: Likely exposure classes for different elements*

Element Likely exposure class

Wall in contact with liquid Cyclic wet and dry – XC3 and XC4 Underside of roofs to

reservoirs Moderately humid environment – XC3 and XC4 Surfaces in contact with

soil (walls and slabs) DC class depending on the aggressiveness of the soil Unprotected surfaces

of walls and roofs Will depend on the circumstances – XC, XD, XS and XF may all be relevant * Exposure classes are in accordance with BS EN 1992-1-1 and BS 8500

Key

Line 1a Tensile strength of the concrete: transitory loading Line 1b Tensile strength of the concrete: sustained loading Line 2a Early-age contraction stress, allowing for creep Line 2b Addition of drying shrinkage stress to line 2a

Structural Design of Concrete and Masonry

caused by loading (as opposed to that caused by restraint of intrinsic deformation such as shrinkage and temperature effects). In such cases σ_cp in expressions (6.2.a) and (6.2.b) will be negative and therefore reduce the shear resistance

• a value of cotθ = 1.0 should be used for the calculation of shear reinforcement as recommended by the Eurocode

Structural design – serviceability limit state Crack widths and watertightness

Table 5 gives the crack width limits and recommendations for the watertightness classes chosen from Table 1. The approach to crack control and the performance implications of the chosen method should be agreed with the client.

Estimation of crack widths

Crack widths are normally calculated for:

• cracking caused by restraint to movement (also referred to as imposed deformations)

• cracking caused by loading

Examples of imposed deformations include early thermal effects, autogenous shrinkage and drying shrinkage.

CIRIA publication C660 contains extremely useful information for the estimation of crack widths. The formulae for crack width calculation are not included here and the reader should consult the reference for fuller details.

Cracking caused by restraint

While cracking is accepted in concrete structures, it is expected to be ‘controlled’ such that cracks will occur at intervals and their width will be small. This requires the presence of a minimum amount of reinforcement in the structural element. Just prior to the occurrence of the first crack, the concrete and the reinforcement will be in tension. At the crack the tension can be carried only by the reinforcement. The minimum reinforcement is calculated such that the reinforcement is able to transfer the tension without yielding. It follows therefore that the stronger the concrete, the greater the amount of reinforcement that will be required to achieve controlled cracking. The tensile strength of concrete at the time when first cracking might be expected is the appropriate strength that should be used. Often, early age cracking at three days is considered critical. Figure 2 shows the relationship between the increase in stress and strength of the concrete over time.

Minimum reinforcement does not guarantee any specific crack width. It is a necessary amount presumed by models for crack width calculations; but not necessarily a sufficient amount to limit the crack widths. Additional reinforcement may well be necessary.

For cracking to occur there has to be restraint to movement. The degree of restraint is another source of uncertainty. Restraint may be internal or external to the element. Internal restraint is caused by differential expansion and will generally be significant in thick sections. External restraint is either end restraint or edge restraint. The

mechanism of crack formation in these two cases is different and this is reflected in the formulae used for the calculation of restrained strain. In the case of members restrained at ends, cracking occurs progressively. Each crack occurs to its full potential width before successive cracks occur. In this case crack-inducing strain is specifically related to the strength of the concrete and the steel ratio.

In the case of members restrained at one edge, the reinforcement and adjacent concrete act as crack distributors and the crack width is a function of the restrained strain rather than the tensile capacity of concrete. See Figure 3 for a typical edge restraint crack pattern. The amount of reinforcement required to limit the crack width to a given value will be considerably higher in the case of members with end restraint compared to those with edge restraint. This is the reason why specifications prohibit casting in alternative bays.

Cracking caused by loading

Procedures set out in BS EN 1992-1-1 may be used to control cracking caused by loading.

Minimising the risk of cracking

BS EN 1992-3 suggests a number of strategies to minimise the risk of cracking. CIRIA publication C660 also provides tips for control of early thermal cracking. The reader should refer to these.

Deflection control

In general, deflections are unlikely to be critical. The procedures in BS EN 1992-1-1, including the span-to-depth formulae, may be used. Where finishes are applied to the structure, manufacturers should be consulted on any limitations on the strains.

Construction Joints

There are essentially two types of joints: those required for minimising

Table 3: Partial factors at ULS for tanks below ground Design situation Verification Permanent actions Loads from contained liquid Earth and groundwater pressures Tank empty Equilibrium 0.90 – 1.50 Strength 1.35 – *

Tank full Strength 1.35 1.35 –

*The more critical values obtained using combinations given in BS EN 1997-111_{should be used here}

Table 4: Partial factors at ULS for elevated towers Design situation Verification Permanent actions Loads from contained liquid Wind Snow

Tank empty Equilibrium 0.90 – 1.50 –

Tank full Strength – walls and base 1.35 1.35 1.50 – Strength – supporting structures and foundations 1.35 1.35 1.50 ψ0 1.50*

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Structural Design of Concrete and Masonry

the risk of cracking (movement joints) and those required for convenience of construction (construction joints). Expansion joints should be provided when reversible movements are expected and contraction joints are suitable when only contraction has to be accommodated.

Joints are potentially vulnerable locations for water penetration. In water-retaining structures, the number of joints should be kept to a minimum. Joints also require maintenance for continued good performance.

It is good practice to protect all joints by incorporating water bars. If the joints are sealed (recommended when another line of defence is deemed necessary), the selection of sealants should be undertaken by a specialist and should take into account:

• the chemical compatibility with other materials or soil with which they are likely to be in contact

• expected movement in the joint • ease of repair and replacement

The size of individual pours will be governed by the site constraints, such as ease of access for concreting and the geometry of the element. The National Structural Concrete Specification12_{recommends default}

pour sizes, but these can be modified if agreed between the designer and the contractor.

At construction joints, structural continuity is required and no relative movement between the sections should occur at the joint. The reinforcement should pass through. The surface of the first pour should be roughened to increase the bond strength and to provide aggregate interlock. Powerful hammers should not be used as they may dislodge the aggregate particles.

Water bars

Water bars are preformed strips of durable impermeable material that are wholly or partially embedded in the concrete during construction. They are located across joints to provide a permanent liquid-tight seal during the whole range of movements. Water bars may be metal strips or proprietary products made of rubber or flexible plastics such as PVC (Figure 4).

At construction joints (both horizontal and vertical), a rigid water bar formed from a strip of black steel (unpainted and non-galvanised) has proved effective. The water bar is placed centrally across the joint. In horizontal joints, the water bar is gently pushed in when the concrete is still green. Separate lengths of the metal water bars need not be welded together. At butt joints between two water bars, a gap should be left equal to aggregate size + 5mm. A cover strip overlapping the two water bars should be placed, again leaving a gap of aggregate size + 5mm.

Figure 3 Edge restraint of wall cast on base

Figure 4 Typical detail of preformed PVC strip backstop water bar

Table 5: Requirements for water-tightness classes*

Tightness class Suggested measures to meet the requirements 0 Structure may be designed using the provisions of clause 7.3.1 _{of BS EN 1992-1-1}

Width of any cracks that can be expected to pass through the full thickness of the section should be limited to w1*

Where the full thickness of the section is not cracked, provisions of clause 7.3.1 of BS EN 1992-1-1 may be used subject to conditions†‡

Cracks that may be expected to pass through the section should be avoided, unless special measures are incorporated (e.g. water bars or liners). There is an implication in the code that it may be adequate to provide water bars to safeguard against leakage through cracks and verify the above-mentioned conditions†‡_{for cracks that do not penetrate the whole depth of the section} 3 Special measures will be required (e.g. liners or prestress)

In document MB Structural Design Compendium May16 (Page 40-45)